As the demand for more powerful microprocessors increases, so must advance the interconnection technology of the integrated circuit (IC). Because an optical signal can travel at a velocity many times faster than an electrical signal, an optical interconnect has the inherent ability to transmit data at higher speeds. In addition, unlike electrical signals, optical pulses are able to cross paths with other optical pulses without interfering with each other, and optical signals are not susceptible to crosstalk. Therefore, it is possible to route a multitude of optical signals in a single interconnect level where multiple interconnect levels would be required for an equivalent electrical circuit. For these and other reasons, optical waveguide interconnects would seem the logical choice to replace electrical interconnects in advanced IC designs.
The simple inverter circuit shown in FIG. 1 is representative of an input buffer for an IC. If the input voltage shown in FIG. 1 is low, P-channel transistor 10 will be turned on while N-channel transistor 11 will be turned off. This will allow the voltage supplied by voltage supply 13 to flow through transistor 10 and appear at the output. This output voltage is then routed to the rest of the circuit. Note that the voltage supply is carried throughout the integrated circuit by power supply line 12. Multiple input buffers, like the one shown in FIG. 1, may then individually tap the voltage off the single power supply line 12 and feed the remainder of the IC. Thus, only a single voltage supply 13 is required to operate the IC. Such a power supply scheme minimizes space and reduces power consumption and complications for both the manufacturer and user of the IC.
To employ an optical interconnection, IC data must first be converted into light pulses before being routed by the optical interconnects. As shown in FIG. 2, an input voltage must trigger laser source 20 to produce the output light. It is this output light which is then routed through an optical interconnect or waveguide to a distant circuit. Upon arriving at its destination, the light pulse may then be reconverted back into a voltage level so that the receiving circuit can manipulate the data in a more conventional manner. A typical advanced IC may have thousands of electrical interconnect lines. If the voltage on each of these interconnect lines must be independently converted to a separate and distinct light signal, thousands of individual laser sources, like the one shown in FIG. 2, would be required to generate the light signals. Such a scheme would be incompatible with current IC technology due the expense and lack of reliability associated with such an arrangement.
To make optical integrated circuits (OIC) feasible, a scheme analogous to the one depicted in FIG. 1 must be employed where the voltage supply 13 of FIG. 1 is replaced with a single laser supply and is routed throughout the OIC as an optical power supply.
FIG. 3 is a schematic diagram indicative of how an optical power supply scheme might be implemented. A single laser 33 is used to supply the light source along optical power supply line 31. Responsive to the input voltage level, optical output line 32 may then tap a small portion of the light source from line 31 without significantly draining the overall optical power available on supply line 31. This is done through railtap 30. In such a manner, multiple input voltages can control multiple railtaps all feeding off a single optical power supply.
A railtap generally comprises a channel of a passive waveguide polymer, a channel of an active waveguide polymer, and electrodes through which to induce an electric field about the active waveguide polymer. For example, in FIG. 3, the rail 31 is constructed of a passive waveguide polymer. A passive waveguide polymer is a material having a relatively stable index of refraction and is capable of channeling light. Output light waveguide 32 comprises an active waveguide polymer. An active waveguide polymer is a material whose index of refraction may be adjusted by inducing an electric field across it. By adjusting the index of refraction, the light path in the material may be redirected. The input voltage of FIG. 3 would be the source of that electric field. By adjusting the index of refraction of waveguide 32 in the vicinity of waveguide 31, a small portion of the light traveling down waveguide 31 can be drawn into waveguide 32 in the region 30. Once in waveguide 32, the light will travel down the length of this waveguide until it reaches the circuit to which waveguide 32 is coupled. At this destination, the light may then be converted back into an electrical impulse for manipulation by conventional IC circuitry.
Because only a small portion of the light in waveguide 31 is tapped into waveguide 32, a substantial amount of the light source still remains in waveguide 31. This remaining light may then be tapped by other railtaps responsive to other input voltages existing along the length of waveguide 31. In such a manner, the single laser supply 33 may be used to supply 100 or more railtaps.
Thus, the need for multiple laser supply sources has been reduced.